The present invention relates to a process for the anodic oxidation of aluminum which is in particular employed as a support material for offset printing plates, the process being performed using an aqueous electrolyte on a basis of phosphoric acid.
Support materials for offset printing plates are provided, on one or both sides, with a radiation-(photo-)sensitive layer (reproduction layer), either directly by the user or by the manufacturers of precoated printing plates. This layer permits the production of a printing image of an original by photochemical means. Following the production of this printing form from the printing plate, the layer support carries the image areas which accept ink in the subsequent printing process and, simultaneously, there is formed, in the areas which are free from an image (non-image areas) in the subsequent printing process, the hydrophilic image background for the lithographic printing operation.
For the above reasons, the following requirements are demanded of a layer support for reproduction layers used in the manufacture of offset printing plates:
Those portions of the radiation-sensitive layer which have become comparatively more soluble following exposure must be capable of being easily removed from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue, and without the developer substantially attacking the support material.
The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water rapidly and permanently during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink.
The radiation-sensitive layer must exhibit an adequate degree of adhesion prior to irradiation (exposure), and those portions of the layer which print must exhibit adequate adhesion following irradiation.
The support material should possess high mechanical strength, e.g., with respect to abrasion, and good chemical resistance to the action of materials such as alkaline media.
The base material employed for layer supports of this type in particular is aluminum. It is superficially roughened by means of known methods, such as dry brushing, wet brushing, sandblasting, chemical and/or electrochemical treatment. Especially the electrochemically roughened substrates are then subjected to an anodizing treatment, during which a thin oxide layer is built up, in order to improve the abrasion resistance. These anodic oxidation processes are usually performed in electrolytes such as H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, H.sub.2 C.sub.2 O.sub.4, H.sub.3 BO.sub.3, amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures thereof. The oxide layers built up in these electrolytes or electrolyte mixtures are distinguished from one another by their structures, layer thicknesses and resistance to chemicals. Aqueous solutions of H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 are predominantly employed in the industrial production of offset printing plates. As far as electrolytes containing H.sub.2 SO.sub.4 are concerned, reference is made, for example, to U.S. Pat. No. 4,211,619 and to the prior art publications mentioned therein.
Aluminum layers produced in aqueous electrolytes containing H.sub.2 SO.sub.4 are amorphous and, in the case of offset printing plates, in general have a weight of about 0.5 to 10 g/m.sup.2, which corresponds to a layer thickness of about 0.15 to 3.0 .mu.m. When a support material anodically oxidized in this way is used for offset printing plates, a disadvantage is presented by the relatively low resistance of oxide layers produced in H.sub.2 SO.sub.4 electrolytes to alkaline solutions. Solutions of this type are employed, to an increasing extent, for example, in the processing of presensitized offset printing plates, preferably in to-date developer solutions for irradiated negative-working or, in particular, positive-working radiation-sensitive layers. Furthermore, these aluminum oxide layers often tend to a more or less irreversible adsorption of substances from the applied reproduction layers, which may, for example, lead to a coloration of the oxide layers, i.e., "staining".
It is also known to perform the anodic oxidation of aluminum in aqueous electrolytes which contain oxygen-containing phosphoric acids and optionally, additional compounds. Processes of this kind are, for example, disclosed in:
U.S. Pat. No. 3,511,661, which describes the use of 42 to 85% strength aqueous H.sub.3 PO.sub.4 solutions at a temperature of at least 17.degree. C. and a current density of about 1.5 to 3 A/dm.sup.2 (direct current), in the production of support materials for printing plates;
U.S. Pat. No. 3,594,289, which describes the use of 5 to 50% strength aqueous solutions of H.sub.3 PO.sub.4 at a temperature of 15.degree. to 40.degree. C. and a current density of 0.5 to 2 A/dm.sup.2 (d.c. or a.c.) for the production of printing plates provided with a reproduction layer that contains a photopolymerizable compound;
German Offenlegungsschrift No. 25 07 386 (British Pat. No. 1,495,861), which describes the use of 1 to 20% strength aqueous solutions of H.sub.3 PO.sub.4 or of polyphosphoric acid, at a temperature of 10.degree. to 40.degree. C., a current density of 1 to 5 A/dm.sup.2 (a.c.) and a voltage of 1 to 50 V, for the production of support materials for printing plates;
U.S. Pat. No. 4,049,504, which describes the use of an aqueous electrolyte with a content of 1 to 3 parts of H.sub.2 SO.sub.4 and of 3 to 1 parts of H.sub.3 PO.sub.4 (total concentration 15 to 25%), at a temperature of 25.degree. to 50.degree. C., a treatment time of 0.25 to 3 minutes and a current density of 1 to 16 A/dm.sup.2 (d.c. or a.c.), for the production of support materials for printing plates;
U.S. Pat. No. 4,229,266, which describes the use of an aqueous electrolyte containing 25 g/l to 150 g/l of H.sub.2 SO.sub.4, 10 g/l to 50 g/l of H.sub.3 PO.sub.4 and 5 g/l to 25 g/l of Al.sup.+3 ions (for example, in the form of Al.sub.2 (SO.sub.4).sub.3.18 H.sub.2 O), at a current density of 4 to 25 A/dm.sup.2 and at a temperature of 25.degree. to 65.degree. C., especially for the production of support materials for printing plates; and
U.S. Pat. No. 4,396,470, which describes the use of an aqueous electrolyte containing from 328 g/l to 380 g/l of H.sub.3 PO.sub.4 in a first anodizing step and the use of another aqueous electrolyte containing from 20 g/l to 150 g/l of H.sub.2 SO.sub.4 and from 250 g/l to 380 g/l of H.sub.3 PO.sub.4 in a second anodizing step, the process parameters including a treatment time for each step of 0.25 min to 4.0 min, a voltage of 15 V to 35 V and a temperature of 15.degree. C. to 46.degree. C.
It is true that the known oxide layers produced in H.sub.3 PO.sub.4 electrolytes often show a greater resistance to alkaline media than oxide layers produced in an electrolyte based on a H.sub.2 SO.sub.4 solution, and that they also present some other advantages, such as brighter surfaces, a better ink-water balance or low dye-stuff adsorption ("staining" in the non-image areas), but they also have some significant disadvantages. In to-date web-processing installations, there can, for example, be achieved oxide layers having weights of not more than about 1.0 g/m.sup.2, the maximum weights being about 1.5 g/m.sup.2, with voltages and bath dwell times commonly employed in industrial practice. It is obvious that layers of such low thicknesses provide a less effective protection against mechanical abrasion than thicker oxide layers prepared in H.sub.2 SO.sub.4 electrolytes. Due to the greater pore volume and pore diameter of an oxide layer built up in a H.sub.3 PO.sub.4 solution, the mechanical stability of the oxide layer itself is reduced, too, which leads to a further decrease of the abrasion resistance. In the case of certain negative-working layers, adhesion problems may also arise so that it is not possible to make universal use of known support materials for printing plates.
By means of the known two-stage oxidation processes, support materials for offset printing plates can be produced which, in respect of practical requirements, exhibit acceptable or even good properties and which also possess a resistance to alkali that substantially comes up to the resistance of an oxide layer produced in an aqueous electrolyte containing H.sub.3 PO.sub.4. These processes, however, necessitate an increased apparatus expenditure, since the anodic oxidation must be performed in two baths, often with an additional intermediate rinsing bath. Such an installation requires supplementary aggregates and control means, which produce, inter alia, further possible sources of error. If H.sub.3 PO.sub.4 is used as the electrolyte in the first state, there is also the danger of "burns" in and on the oxide layer, which lead to pinholes which, especially in the field of lithography, are very undesirable. There have also been disclosed mixed electrolytes with a constant of H.sub.3 PO.sub.4 and at least one further component, in particular an aqueous mixed electrolyte with a content of H.sub.2 SO.sub.4, H.sub.3 PO.sub.4 and Al.sup.+3 ions, but this electrolyte, too, results in oxide layers exhibiting a low resistance to alkaline media, which will be demonstrated by the comparative examples below.